Try removing the media card. If that is not it, see if you can boot in safe mode. KB17877-How to start a BlackBerry smartphone in Safe Mode
If you can boot in safe mode at least you will be able to back up. Do that first. Then delete the last few apps you installed and do a battery pull. Or, you may just want to reinstall the OS.
Deleting problem apps may be the fix. If it were me I would reinstall the OS, unless the problem turns out to be the media card.